If you can play it as it is, I'd do that. If it is a problem, file the nut. Shimming the neck up will affect every string, and sustain. You aren't going to get a desired result from it.filing the nut will drop the string on the first 5 frets slightly. I'd be very careful not to take much off. It's kind of hard to add nut that isn't there. Guy above me mentioned filing the bridge. Do that instead. Worst case, you can get a new bridge saddle.